Process for coating substrates in strip-form with photographic emulsion

ABSTRACT

A process for coating a viscous solution by a wetting process on a movable substrate. The coating solution is formed by a narrow gap between coating apparatus and the movable substrate so that the narrow gap is part of a coating chamber of the coating apparatus. The coating chamber is connected to a vacuum means at the side of the coating apparatus from which the substrate is introduced.

' United States Patent Herzhoff et al.

[ 1 Feb. 29, 1972 [54] PROCESS FOR COATING SUBSTRATES IN STRIP-FORM WITHPHOTOGRAPHIC EMULSION lnventors: Peter Herzhoff, Leverkusen; Hans Grei,Cologne-Stammheim; Fritz Maus, Cologne- Flittar'dl" M WJBEng SEEweEEeY;-Wiili Wasser, both of Leverkusen; Kurt Browatski, Lagenfeld; JosefFriedsam, Opladen, all of Germany Assignee: Agfa-GevaertAktiengesellschaft, Leverkusen, Germany 1 Filed: Feb. 10, 1969 Appl,No.: 797,808

U.S.Cl ..117/34,117/61, 117/119, 118/50, 118/410 Int. Cl. ..B44d 1/12,B05c 3/l2, G03c 1/74 Field ofSearch ..117/34, 1 19,61; 118/50, 63,118/410 References Cited UNITED STATES PATENTS 6/1954 Beguin ..1 17/349/1956 RusSelL... ..1 18/410 9/1956 Russell.... ...1 18/410 X 9/1960Nadeau... ..1 17/34 9/1965 Miller ..l l7/34 1/1966 Bartlett... ..l18/63x 10/1967 Good ..1l8/50x 3/1970 lshiwata et al. 17/34 3/1970 lshiwata eta] ..1 18/50 Primary Examiner-Murray Katz Assistant Examiner-William R.Trenor Attorney-Connolly and l-lutz ABSTRACT A process for coating aviscous solution by a wetting processon a movable substrate. The coatingsolution is formed by a narrow gap between coating apparatus and themovable sub strate so that the narrow gap is part of a coating chamberof the coating apparatus. The coating chamber is connected to a vacuummeans at the side of the coating apparatus from which the substrate isintroduced.

2 Claims, 2 Drawing Figures PAIENTEUFEB 2 9 I972 INVENTOR will i MIMI,

Peter.Herzholfi', 11:111.: Zn- 2f, Fri M1 Kurt Browatzki.

PROCESS FOR COATING SUBSTRATES IN STRIP-FORM WITH PIIOTOGRAPIIICEMULSION Reference is made to U.S. application Ser. No. 798,118 filedFeb. 10, 1969; Ser. No. 797,809 filed Feb. 10, 1969 and Ser. No. 797,800filed Feb. 10, 1969.

This invention relates to a process for coating substrates in strip formwith viscous solutions, preferably with photographic emulsions, in whichthe substrate is wetted in a sealed coating chamber whose end at thepoint where the emulsion emerges is formed by a narrow gap several timeswider than the wet finished layer is thick, and in which to counteractthe forward movement of the moved substrate, layer formation takes placeunder a adjustable static pressure differential between the inlet endand the outlet end of the gap, the lower pressure prevailing at theinlet end.

It is known that the negative pressure can be generated by evacuatingthe interior of the supply vessel, for example by means of a pump. Thelevel of the coating solution in the supply vessel is kept constant by alevel meter or gauge which acts on a pump dispensing the coatingsolution.

One disadvantage of this embodiment is that the vacuum created by thepump is subject to fluctuations, in addition to which air can leak atone place or another. Separating of vortices increase fluctuations ofpressure. Accordingly this conventional coating process is not alwaysreliable in operation. The method of coating in strip-form can becarried out in techniques such as described in copending applications,U.S. application Ser. No. 798,1 l8 entitled A Process for CoatingSubstrates in Strip-Form and U.S. application Ser. No. 797,800 entitledA Process for Coating Strip-Form Substrates," both filed Feb. 10, 1969.

The object of the invention is to provide a coating process guaranteeingan exact, consistently uniform application of the coating solution tothe substrate in a layer thickness with extremely narrow tolerances. Inthis case the whole solution which is supplied is applied to the stripwithout return motion. The smallest distance between the coating deviceand the strip to be coated should be larger than two times the thicknessof the coating layer.

According to the invention, this object is achieved'by adjusting therequired negative pressure through dropping the level ofthe contents ofthe supply vessel in relation to the position of the coating gap. Inthis way it is very easy to adjust layer thickness.

The fall in the level of the content of the supply vessel, i.e., thelevel of the coating solution, is most readily obtained throughconsumption. In another embodiment, which enables the level to beadjusted more quickly, a corresponding quantity of coating solution isrun off from the supply vessel or otherwise removed. Alternatively, theentire supply vessel together with its contents is vertically adjustablerelative to the coating gap.

Coating processes which do not provide a volumetrical control of thethickness of the layer, show the following disadvantage: A stablerelation of mechanical data of coating to the static vacuum and to thethickness of the layer resulting therefrom exists only, if the flowproperties of the coating solution are always constant. This conditioncannot be fulfilled when using different solutions. Even when using thesame solution, a change of the flow properties during periods ofnonoperation of the device cannot always be avoided. The production of acoating layer of constant thickness therefore requires the level ofsolution in the vessel to be raised or lowered proportionally to thealterations of the flow properties of the solution; thus, it isnecessary, either to measure the alterations of the thickness of thelayer and prevent such alterations by controlling the level, or todetermine the alterations of the flow properties of the solution and,proportionally thereto, to control the level in the vessel so as togarantuee the desired thickness all over the operating time by theexperimentally determined relation between the flow properties, heightof level in the vessel and the resulting thickness of layer. Bothmethods require a large expenditure of measuring and controllingequipment. The second method nevertheless does not garantuee a result ofsufficient exactness.

- substrate to be coated.

In this way, the static pressuredifferential and hence the level ofliquid in the supply vessel are automatically adjusted in dependenceupon the flow properties of the coating solu tion in such a way thatexactly the same amount of coating solution is applied to the substrateas is dispensed per unit of time. Any difference between the quantity ofcoating solution applied and the quantity applied produces a change inlevel in the supply vessel which affects the quantity applied in such away that theaforementioned difference is eliminated. The arrangement isself-regulating. The level of coating solution in the supply vesselremains freely adjustable. It is of advantage to keep the cross sectionof the supply vessel small. For example the supply vessel may consistsolely of a standpipe.

If subsequently there is a change in the flow properties, the level inthe supply level will'in fact change, although the resulting layerthickness will remain unchanged, as required because the rate ofapplication remains equal to the rate at which coating solution isdispensed. In addition, it is readily possible-to ensure that thethickness of the coating applied remains constant in the event of anychange, in the rate at which the substrate to be coated travels forward.

To this end, the quantity of coating solution dispensed is alteredproportionally to the rate of travel. According to the invention, thisis preferably. done by designing the dispensing or metering device inthe form of a pump whose throughput is proportional to its rate ofrevolutions and whose drive is coupled at a fixed rotational speedration with the drive of the substrate.

The process of the invention as described before, has proved to beefficient in practice. 1

However, when starting the process or restarting after an interruptionof coating, the coatings obtained have a higher thickness than desiredduring the time the level in the vessel needs for lowering. On accountof the high speed of coating and the long adaptation time of the levelof the solution, which takes about one minute, a larger portion of thestrip is coated in a thickness of layer beyond the tolerance limits.These portions of the strips are rejected.

According to a preferred embodiment of the process, the portions of thestrip which are not useful because the coating thereon is relativelythick, can be decreased in length to a few inches by supplying thevolumetrically fed coating liquid to the coating chamber through acompletely sealed pipe system which is equipped with an air cushion,preferably enclosed in the vessel.

In a further development of the process according to the invention forapplying coatings in several superimposed layers, the strip-formsubstrate is coated from separate coating chambers in a common coatingblock, wherein the sealed coating chambers with the correspondingcoating gaps are arranged without interspace one after another beingseparated from one another by a common sealing wall.

In addition, the invention provides in known manner for the applicationof a vacuum at the inlet end of the coater so that the negative pressurein the coater can be maintained. A blocking vacuum is preferably appliedat the end of the coating gap, too.

Two embodiments of the process according to the invention and theassociated apparatus are described by way of example in the followingwith reference to the accompanying drawings, wherein:

FIG. 1 is a cross section through a one-layer coater,

FIG. 2 is a cross section through a multilayer coater.

As shown in FIG. 1, a strip 1 is guided over a roller 2 and passes arear wall 3 of a coating block 4. In a coating chamber 5, the strip 1 iswetted with coating solution. A front coating wall 6 in conjunction withthe roller 2 and the strip 1 formsthe actual coating gap 7 which isresponsible for determining the thickness of a layer 8 on the strip 1.The coating chamber communicates through a pipe 9 with a supply vessel10. After commencement of the process the level of the coating solutionin the supply vessel 10 is adjusted relative to the level of the coatinggap 7 by lowering it the required extent Ah, generating a correspondingnegative pressure. The vessel 10 is enclosed so as to provide the aircushion mentioned above. The reference 11 denotes the device fordispensing the coating solution. The rear wall 3 of the coater isadjoined by a vacuum trap 12 in a direction opposite to that in whichthe strip travels. A chamber 13 of the vacuum lock 12 is connected bymeans of a suction socket 14 to a suction fan (not shown). The chamber13 is preceded by another chamber 15 in order to form a kindoflabyrinth.

In FIG. 2, the multilayer coater consists of a guide roller 21 for astrip 22. A coating block 23 is provided with two coating chambers 24and 25 separated from one another by an intermediate wall 26. A firstemulsion coating solution is fed through a connecting socket 27 to thecoating chamber 24, being applied to the strip 22 in the form ofa layer29 through a coating gap 35. Another coating solution is fed to thesecond coating chamber 25 through a connecting socket 30, being appliedto the first emulsion layer 29 in the form of a layer 31 through thecoating gap 28. Arranged at the inlet end of the coating block 23 thereis a vacuum lock 32 whose vacuum chamber is connected to a suction fan(not shown) through a vacuum pipe 34.

We claim:

1. In a process of applying a coating of photographic emulsion ofuniform and controllable thickness to a moving foil,

the foil being wetted in a body of photographic emulsion coatingsolution, the body being subjected to a hydrostatic pressure difference,which is formed by the moving foil itself by picking up the coatingsolution from the body of the solution at the lower pressure prevailingwhere the foil is first wetted by the body, the improvement whichcomprises confining the body of photographic emulsion coating solutionto a gap, which is several times wider than the thickness of the coatingon the foil, feeding the coating solution from a coating solutionsupply, which is in a completely sealed self-communicating pipe system,to a gap between the moving foil and an adjacent surface of a'chamber ofthe system, removing the coating solution from the gap onto the movingfoil, lowering the level of the coating solution in said supply by saidremoval of the coating solution and creating a pressure differentialwithin said gap between the side closest to the chamber and the sidewhere the moving foil departs from the gap, metering the ad ding ofcoating solution into said supply under an air cushion above the levelof the solution in the sealed system at a rate variably proportional tothe rate of travel of the moving foil so that the rate of volumetricaddition is increased with an increase in the rate of travel, per unitof time, and thereby automatically adjusting the level of the coatingsolution in the supply and adjusting the static pressure differentialbetween said sides of the gap.

2. A process for coating several layers according to claim 1, whereinthe different layers are applied successively via a plurality ofself-communicating pipe systems, each system having its free adjustablecoating solution level in a different supply vessel, the supply vesselsbeing provided with an air cushion, and metering each coating solutionto each supply vessel.

2. A process for coating several layers according to claim 1, whereinthe different layers are applied successively via a plurality ofself-communicating pipe systems, each system having its free adjustablecoating solution level in a different supply vessel, the supply vesselsbeing provided with an air cushion, and metering each coating solutionto each supply vessel.